1. Field of the Invention
The present invention relates to a print device wherein a hammer bank forms an image on a recording medium while reciprocally transported by a shuttle mechanism. The present invention more particularly relates to such a print device including a reversing urging means for reversing a transporting direction of the hammer bank and to a method of controlling reciprocal movement of the hammer bank.
2. Description of Related Art
There has been known a print device including a hammer bank for forming an image on a recording medium, such as a sheet of paper, while reciprocally transported. Dot line printers and shuttle printers are representative examples of such print devices. Several types of shuttle mechanisms are known for reciprocally transporting the hammer bank. For example, one type of mechanism is provided with a cam or a link mechanism for converting rotational drive of a drive motor into a linear movement. Another type of mechanism reverses a transport direction of the hammer bank by changing rotational direction of a drive motor. There is also known a direct drive type mechanism including a linear motor. The direct drive type mechanism requires no transmission mechanism for transmitting drive of the linear motor to the hammer bank.
In order to provide a print unit wherein printing is performed in a improved speed, there has been proposed a shuttle mechanism provided with urging means, such as a spring. The urging means facilitates acceleration of transport speed of the hammer bank when its transport direction is reversed.
FIG. 1 shows an example of printing unit of a print device. The print unit includes such a shuttle mechanism provided with springs. Specifically, as shown in FIG. 1, the printing unit 1 includes a shuttle mechanism 2, a hammer bank 3, a sensor 4, and a shuttle drive mechanism. The shuttle mechanism 2 includes a guide shaft 11, direct drive bearings 12, a linear motor 20, an inversion mechanism 30, and springs 40. The shuttle drive mechanism includes a controller 50, a shuttle control circuit 60, and a shuttle drive circuit 70. The guide shaft 11 extends leftward and rightward as viewed in FIG. 1. The direct drive bearings 12 are reciprocally movably mounted on the guide shaft 11. The hammer bank 3 is supported on the direct drive bearings 12, and so reciprocally movable with the direct drive bearings 12. Although not shown in the drawings, the hammer bank 3 is provided with a plurality of printing hammers for forming a dot pattern on a recording medium based on print data received from an external device. The linear motor 20 is provided with a coil 21 and magnets (not shown), and driven in a well known manner. Although not shown in the drawings, the coil 21 includes a reverse coil and a constant velocity coil. The inversion mechanism 30 has a pair of timing pulleys 32 and a timing belt 31 wound around the timing pulleys 32. The coil 21 is connected to the direct drive bearings 12 via the inversion mechanism 30. With this configuration, the drive force of the linear motor 20 is transmitted to the direct drive bearings 12 so as to reciprocally transport the direct drive bearings 12. The coil 21 is also reciprocally transported in synchronization with the direct drive bearings 12, but always in a direction opposite to the direction in which the direct drive bearings 12 are transported. In this way, the coil 21 serves as a counter balance. That is, when the direct drive bearings 12 with the hammer bank 3 mounted thereon are reciprocally transported, such a reciprocal movement of the coil 21, which has a fixed weight, achieves leftward and rightward weight balance of the print device, thereby reducing vibration generated on the print device due to the transport of the direct drive bearings 12.
As shown in FIG. 1, the springs 40 are disposed at each end of the guide shaft 11 and the coil 21 for supplying repulsive force to the hammer bank 3, via the direct drive bearings 12, and the coil 21 when their transport directions are changed during reciprocal transport. The sensor 4 is provided near a movable portion, which in the present example is on the hammer bank side, for detecting a position of the hammer bank 3. The shuttle drive circuit 70 energizes the coil 21 by supplying an electric current, and the shuttle control circuit 60 controls the amount of electric current supplied to the coil 21. Based on positional information supplied by detection by the sensor 4, the controller 50 controls the shuttle control circuit 60 and the shuttle drive circuit 70 to move the hammer bank 3 in a predetermined shuttle speed pattern which is graphically shown in FIG. 3. The controller 50 also receives a variety of commands from an external device (not shown).
FIG. 2 shows a sheet transport mechanism 80. A platen 81 is rotatably supported on a printer frame (now shown). A pair of left and right pin tractors 82 are provided for transporting a sheet S on the platen 81 in a direction perpendicular to the reciprocal movement direction of the hammer bank 3. The platen 81 and the pin tractor 82 are driven by a sheet feed motor 83. An ink ribbon 84 is provided for supplying ink.
As shown in FIGS. 3 and 4, the hammer bank 3 is reciprocally moved in a transport region defined by a pair of predetermined reversing positions P0. The transport region is divided into a constant velocity region and two reverse regions. The constant velocity coil and the reverse coil are energized in the constant velocity region and in the reverse regions, respectively. In the constant velocity region, the hammer bank 3 is transported at a constant speed. On the other hand, in the reverse regions, the hammer bank 3 abuts against the spring 40 and influenced by the repulsive force of the spring 40. That is, in the reverse regions, the repulsive force is generated, and deceleration and acceleration of the shuttle are performed.
More specifically, when the hammer bank 3 enters the reverse region from the constant velocity region, the hammer bank 3 is decelerated by pressing against the spring 40. Then, the velocity of the hammer bank 3 drops to zero at the reverse point P0 wherein the spring 40 is maximally compressed. At this point, the repulsive force of the spring 40 increases to its maximum, and the transport direction of the hammer bank 3 is reversed. Next, the repulsive force of the spring 40 starts accelerating the hammer bank 3 in the reverse direction.
In the above-described print unit 1, there is a need to perform initialization operations when the print unit 1 is first started. The initialization operations mean repeating reciprocal transport, that is, shuttle operations, of the hammer bank 3 not associative with printing operations until a predetermined shuttle speed is achieved. More specifically, when the printing unit 1 is driven, the shuttle operations are started. However, the predetermined shuttle speed cannot be reached immediately. Therefore, the shuttle speed is gradually increased by repeating shuttle operations using the repulsive force of the spring 40. As the hammer bank 3 accelerates, the amount that the hammer bank 3 compresses the spring 40 increases. Printing is started once the hammer bank 3 compresses the spring 40 by a predetermined amount and the shuttle speed reaches a predetermined shuttle speed.
The initialization operations are also performed when shuttle operations are restarted after shuttle operations are temporarily stopped during printing.
As shown in FIG. 4, the shuttle operations during printing are repeated at a substantially fixed cycle. Normally, sheet feed operations are performed while the hammer bank 3 is in one of the reverse regions. That is, after a single row's worth of printing is completed at a position P1, sheet feed operations are performed for a single line's distance and completed by the time the shuttle reaches a print start position P2. Herein after, sheet feed operations for transporting the sheet a single line's distance will be alternatively referred to as a carriage return. Then, printing for a subsequent row is started. However, printing operations do not exclusively involve printing single lines separated by a single carriage return. Sometimes, sheet feed operations are required for a consecutive plurality of carriage returns. In this case, the amount of the sheet that must be fed at one time is greater than that during a single carriage return. Therefore, sheet feed for a plurality of carriage returns requires a greater amount of time than for a single carriage return. This relationship can be expressed by the following formula:
Tf&lt;Tfn PA1 wherein, Tf is the time duration required for a single carriage return; and PA1 Tfn is the time duration required for a plurality of carriage returns (n is an integer equal to two or more). PA1 Tf&lt;Tfn&lt;(Tf+Tp), PA1 the wasted time Tw is calculated by the following formula: PA1 Tw=(Tf.times.n+Tp)-Tfn
For this reason, sheet feed operations for a plurality of carriage returns may not be completed while the hammer bank 3 is in a reverse region, that is, while the hammer bank 3 is between the position P1 and the print start position P2. Therefore, after the printing for one row's worth of images is completed at the position P1, it is desirable to stop the hammer bank 3 at the print start position P2 and restart the shuttle operation in synchronization with completion of the sheet feed operations. However, in this case, the hammer bank 3 cannot be accelerated to shuttle speed and cannot reach the predetermined shuttle speed immediately after the shuttle operations are restarted, because once the shuttle is stopped at the position P1, the repulsive force of the spring 40 cannot be utilized to accelerate the hammer bank 3. In order to utilize the repulsive force of the spring 40, the hammer bank 3 should be located at the reverse position P0. Accordingly, the initialization operations must be again performed, and printing for a subsequent row cannot be performed immediately.
In order to overcome these problems, when the plurality of carriage returns are not completed by the time the shuttle passes the print start position P2, the shuttle operations are continuously performed. Then, printing of the subsequent row is started at a next available print start position, for example, at a print start position P3, instead of at the print start position P2. That is, printing is not performed during printing region Tn. Therefore, wasted time is generated and printing efficiency is decreased.
For example, when Tfn is the time required to feed a sheet a plurality of lines (carriage returns) distance and Tp is the time duration required for one way transport of the hammer bank 3, and if the following relationship is established:
In other words, the amount of wasted time increases proportionally to the number of plural carriage returns performed during printing operations. As a result, operation efficiency is reduced.
Further, there have been known printers including a plurality of different printing modes relating to different shuttle speed patterns. However, in order to change the shuttle speed patterns during printing operations, the shuttle operations must be temporarily stopped. Therefore, in the conventional configuration described above, it is difficult to quickly change the shuttle speed pattern during printing operations.